1. Field of the Invention
The invention relates to a pulse width modulator, more particularly to a noise-resistant pulse width modulator.
2. Description of the Related Art
A conventional pulse width modulator 1, as shown in FIG. 1, includes a first differential amplifier 131, a second differential amplifier 132, and an RS flip-flop 14. The conventional pulse width modulator 1 is operable so as to provide a pair of complementary control signals (Vc1, Vc2), each of which controls on and off operations of a respective one of first and second switches 21, 22 of a passive network 2 such that a feedback voltage signal (Vfb) is obtained from the passive network 2. The first differential amplifier 131 has a non-inverting input terminal that receives a first reference voltage signal (Vref1), an inverting input terminal that receives the feedback voltage signal (Vfb), and an output terminal that provides a first differential voltage signal (Vp) corresponding to the first reference voltage signal (Vref1) and the feedback voltage signal (Vfb). On the other hand, the second differential amplifier 132 has an inverting input terminal that receives a second reference voltage signal (Vref2), a non-inverting input terminal that receives the feedback voltage signal (Vfb), and an output terminal that provides a second differential voltage signal (Vr) corresponding to the second reference voltage signal (Vref2) and the feedback voltage signal (Vfb).
The RS flip-flop 14 has an S input that receives the first differential voltage signal (Vp), an R input that receives the second differential voltage signal (Vr), and Q and Q′ outputs, each of which provides a respective one of the complementary control signals (Vc1, Vc2).
In operation, with further reference to FIGS. 2(a) to 2(d), when the voltage level of the sawtooth feedback voltage signal (Vfb) is lower than that of the second reference voltage signal (Vref2), the first differential voltage signal (Vp) sets the RS flip-flop 14 such that the complementary control signals (Vc1, Vc2) control operations of the first and second switches 21, 22 in the on and off states, respectively. On the other hand, when the voltage level of the feedback reference voltage (Vfb) is higher than that of the first reference voltage signal (Vref1), the second differential voltage signal (Vr) resets the RS flip-flop 14 such that the complementary control signals (Vc1, Vc2) control operations of the first and second switches 21, 22 in the off and on states, respectively.
Although the conventional pulse width modulator 1 achieves its intended purpose, in an actual operation, noise is present in the feedback voltage signal (Vfb) The noise in the feedback voltage signal (Vfb) causes erratic behavior of the conventional pulse width modulator 1, as illustrated in FIGS. 3(a) and 3(b). That is, when the level of the noise-affected feedback voltage signal (Vfb) becomes higher than that of the first reference voltage signal (Vref1), or lower than that of the second reference voltage signal (Vref2), the RS flip-flop 14 may be erroneously set or reset by the first or second differential voltage signal (Vp, Vr). As such, the complementary control signals (Vc1, Vc2) control operations of the first and second switches 21, 22 in an unpredictable manner. Furthermore, noise may also appear in one of the first and second reference voltage signals (Vref1, Vref2). This, too, contributes to the erratic behavior of the conventional pulse width modulator.